The present invention relates to a position sensor and to parts therefor. The invention has particular although not exclusive relevance to x-y digitising tablets which operate with a cordless stylus. The invention is particularly useful for embedding behind the display of a hand-held electronic device such as a personal digital assistant (PDA), mobile telephone, web browser or products embodying combinations of these.
U.S. Pat. No. 4,878,553 discloses an x-y digitising tablet which uses a resonant stylus. The digitising tablet comprises a large number of overlapping but separate loop coils which are arrayed in the x- and y-direction. These loop coils are connected through a switching matrix to an excitation circuit and a receiving circuit. The system is arranged so that each of the loop coils is firstly connected to the transmitting circuit and then to the receiving circuit. The system identifies the current position of the stylus by detecting the excitation and sensor coil combination which provides the greatest output signal levels. A more accurate determination of the stylus position is obtained by performing a quadratic-type interpolation using the signals received from the loop coils adjacent to the loop coil having the maximum signal level. One problem with this system is that there is a trade-off between the system's response time and the accuracy of the tablet. In particular, for high accuracy, a large number of overlapping loop coils are required, however, as the number of loop coils increases, the system's response time decreases. Another problem with this system is that it has a relatively large “dead band” at the edge of the overlapping loop coils which causes problems when space is critical such as in hand-held computer devices such as PDAs and mobile telephones.
Another problem with this system is that it is too expensive for embedding into hand-held portable communication and information devices targeted at consumer markets. The conventional approach is to use a resistive touch screen in these applications. This touch screen is usually placed over the liquid crystal display of the device. However, the problem with using such resistive touch screens is that it degrades the LCD's contrast, it increases glare and reflections from other light sources, it suffers from wear and adds significantly to product thickness. Further, it cannot cope with unintended contact with the touch screen because it does not differentiate between contact from different objects. As a result, the user cannot rest their hand on the device when using it, which may prevent them from using their normal comfortable writing style.
The applicant has described in its earlier International application WO98/58237 an alternative electromagnetic x-y digitising system which is more suited to such consumer applications. This system uses a set of periodic sensor windings and an excitation winding which can be embedded behind the liquid crystal display of the device. In operation, the excitation winding energises a resonant stylus which in turn induces signals in the sensor windings. Due to the periodic nature of the sensor windings, the output signals from them periodically vary with the relative position between the stylus and the sensor windings. Because this system only requires a few windings, there is not the same trade-off between system accuracy and response time (since all the windings can be read in parallel) and the “dead band” at the edge of the board can be made smaller because there are fewer windings. Further, because there are fewer windings, the sensor requires fewer connections to be made to the processing electronics and the system requires less power because only a single excitation winding is used which can be optimised for high efficiency by increasing the amount of copper and distributing it at the edge of the board without using excessive space at the edge of the board.
As acknowledged in this earlier International application, it is desirable to maximise the number of turns of the sensor windings and the excitation winding in order to maximise the coupling between those windings and the resonant stylus and to allow the suppression of unwanted spatial harmonic signals. One problem with the system described in the Applicant's earlier International application is that the windings are formed by wire bonding technology which suffers from poor resolution (for example three wires per millimeter) which limits the number of turns which can be formed for a given sensor area. Another technique which can be used which can increase the resolution is the use of printed circuit boards. However using printed circuit boards causes the problem of designing the windings in order to minimise the number of printed circuit board layers which are required and to minimise the number of through connections or vias which are required between the layers, in order to reduce manufacturing costs and wasted areas of the board especially at the edges.
A further problem which is common to both these known electromagnetic x-y digitising systems is that a positional error is introduced into the measurements if the stylus is tilted relative to the digitising tablet. In the applicant's above mentioned International application, they have proposed a technique for processing the signals output by the periodic sensor windings in order to reduce the effect of stylus tilt. Similarly, the proprietor of U.S. Pat. No. 4,878,553 has described, in EP-A-0680009, a technique for processing the signals received from the overlapping loop coils in order to reduce the effect of stylus tilt. However, these techniques require the reading of the signals from more of the sensor coils and the subsequent processing of these signals in order to determine the tilt measurement, which therefore increases the time required to determine a position measurement.
Another problem with the electromagnetic x-y digitising systems described above is that if they are to mimic the action of a conventional pen, then they must be able to detect when the stylus is pressed against the writing surface. This is usually accomplished by designing the stylus so that its resonant frequency changes with pressure applied to the tip of the stylus. This leads to problems of how to design the resonant stylus so that the system can reliably detect “stylus down” whilst providing a stylus which feels natural to use. Several prior art designs of this type of stylus are disclosed in U.S. Pat. No. 5,565,632. However, these systems suffer from problems of ensuring that a desired and repeatable resonant frequency change is obtained when pressure is applied to the stylus.